A crucial element in the development of effective therapeutic and prophylactic strategies for AIDS is an experimental animal model in which course of immunodeficiency virus infection parallels the pathogenesis of the human disease. SIV infection of macaques is such a model since this virus induces an immunodeficiency syndrome in infected macaques that is remarkably similar to human AIDS. Therefore, candidate vaccines can be evaluated not only for their ability to prevent infection but also for their ability to prevent AIDS. The major vaccine effort within the laboratory has been an evaluation of the highly attenuated vaccinia virus Ankara (MVA) strain as a recombinant vector used for priming that is followed by a subunit SIV boost. Interest in this approach is based upon a pilot study of 12 macaques in which animals immunized with the MVA-SIV recombinant exhibited significant down modulation of virus replication that was associated with prolonged survival.